1. Field of the Invention
The present invention relates to a compact imaging head, a high speed multi-head laser imaging assembly comprising a plurality of such heads, and a method of imaging heat or light sensitive media using such an assembly. In particular, the assembly comprises a plurality of compact imaging heads (referred to as modules when they are interchangeable) which operate in unison to direct radiation from groups of laser emitters to modulators. The assembly and method of the present invention are capable of directing radiant energy produced by each module for imaging heat or light sensitive media such as a printing plate.
2. Background Information
Some of the current trends in the thermal offset printing plate industry have been in the area of increased productivity, especially as they relate to so-called xe2x80x9cComputer to Platexe2x80x9d (CTP) systems. However, such conventional systems are presently limited, especially as they relate to imaging of thermal offset plates. Conventional internal drum systems are limited, for example, with respect to the spinning speed of the mirror, the commutation time on/off of the laser beam (for acousto-optic modulators with YAG lasers, red and UV laser diodes and optical fiber lasers), and power of the laser sources. Conventional external drum systems which have a plurality of laser sources such as diodes are limited, for example, with respect to respective rotational speeds, respective number of diodes and the total power generated thereby. Conventional external drums employing a spatial modulator also have power limitations as well as limitations with respect to the number of spots produced thereby. Conventional flat bed systems have xe2x80x9cwidth of platexe2x80x9d limitations, resolution limitations, as well as limited scanning speeds, modulation frequencies and power of the respective laser source.
A conventional system in which a laser beam is widened in one dimension to cover an array of a substantial number of electro-optic gates (so that a large number of adjacent spots can be formed and thus constitute a xe2x80x9cwide brushxe2x80x9d) is described in U.S. Pat. No. 4,746,942, which is incorporated herein by reference. In particular, this patent discloses that the beam is divided by the gates into a plurality of potential spot-forming beams. The transmission of each beam to a photosensitive surface for imaging is selectively inhibited in accordance with a pre-determined pattern or program, while the beams are swept relative to the photosensitive surface to form characters and other images.
However, the number of spots of the brush described in this patent may be limited by optical aberrations. In addition, the power that a single laser source can produce limits the imaging speed of thermo-sensitive plates because of their low sensitivity. The performance of a spatial modulator with a single laser source can also be limited. Conventional xe2x80x9cbrushxe2x80x9d systems generally use spatial modulators such as, e.g., electro-optic ferro-electric ceramic (PLZT) modulators, total internal reflection (TIR) modulators and micro-mirrors, are similarly limited.
TIR modulators based on the use of LiNbO3 crystals are of particular interest because of their commutation speed. This type of modulator is described in the literature and several patents such as in U.S. Pat. No. 4,281,904, which is incorporated herein by reference. However, for the imaging of thermo-sensitive plates where a high level of energy is necessary, the crystal is submitted to a strong energy density that induces photorefraction effects which negatively affect the operation of the modulator. These effects, known as xe2x80x9coptical damage, dc driftxe2x80x9d limit the amount of energy which can be handled.
An imaging xe2x80x9cheadxe2x80x9d comprising a source of laser energy, associated optics, and a modulator capable of generating a line segment or xe2x80x9cbrushxe2x80x9d is described in co-assigned U.S. Pat. No. 6,137,631, which is incorporated herein by reference. Such a module or head typically projects a thin (i.e. 12 micron) line-segment or brush having a width of 5.2 mm (i.e. a 256 pixel line segment). The imaging productivity of an imaging system is disadvantageously limited by the small size of such a line-segment.
One of the objects of the present invention is to overcome the limitations and disadvantages of the above-described conventional CTP systems by increasing their productivity. Another object of the present invention is to increase the number of spots generated using a laser beam by juxtapositioning the brushes produced by a plurality of compact imaging heads such that each head produces several hundreds light spots. Thus, the available power and the pixel rate of conventional CTP systems can be multiplied by the number of heads provided in the assembly and method of the present invention. It is another object of this invention that the system of this invention may be employed in internal and external drum systems, as described above, as well as in flat bed platesetter systems, such as described in WO 00/49463, the entire disclosure of which is incorporated herein by reference. It is yet another object of this invention to provide a compact imaging head which may be employed in the assembly and method of this invention, where it is also referred to as a xe2x80x9cmodule.xe2x80x9d
It is one feature of this invention that the brushes of light produced by each module in the head assembly are controlled to provide a continuous scan line which is the aggregate of the individual brushes emitted from each head, thereby avoiding any gaps in the overall scan line employed for imaging. It is another feature of this invention that the width, orientation, shape, power and timing of each brush is controlled to permit the aggregate of individual brushes to be employed as a continuous scan line. The system and method of this invention thus advantageously are able to overcome the limitations of existing xe2x80x9csingle headxe2x80x9d systems which are usually limited to small (e.g. 256 pixel) line segments. Other objects, features and advantages of the system and method of this invention will be apparent to those skilled in the art.
Several optical heads are mounted on a common carriage adapted to scan a photosensitive printing plate. Each head is equipped with a laser source, a modulator and projection optics and can project an image (i.e. xe2x80x9cbrushxe2x80x9d) of the active zone of the modulator containing a plurality of pixels. The optical track of beams in each head is folded several times in such a way as to reduce the width of the head. When the carriage moves from one edge of the plate to the other edge a swath of pixels is projected as if painted by the brush. Each head includes means to adjust the height, spatial position, orientation and intensity of the brush it generates. Each head is accurately positioned on the carriage so that at least two abutting swaths are projected during each sweep of the carriage to produce a wider swath. The carriage generates pulses indicative of its position relative to the location of the plate edges. Each head is capable of receiving a signal to time the projection of brushes. The relative movements between the carriage and the photosensitive plate are controlled by electronic means.